(Second) Awakening
Right at the start of the infinity arc, Sailor Uranus, Sailor Neptune, and (later) Sailor Pluto are hell bent on preventing Saturn's coming by killing Hotaru. They don't enlighten Sailor Moon about their plan until they realize that they were always meant to work together. Then, even when Sailor Moon and the Sailor Guardians protest the killing of Hotaru, they still decide to go ahead with their plan.
Since they know of the power of their talismans, they are determined to prevent it. Ironically, they gather together even though the line "You must not gather the three talismans" is repeated over and over. By coming together, the possibility of calling upon Saturn was just that much higher, even if they weren't sure the talismans would activate. In the anime, Uranus and Neptune aren't aware of their power of the talismans and the talismans really only serve to summon the Holy Grail. Interestingly enough, the talismans summon the Holy Grail, which gives Sailor Moon immeasurable power, and they also summon Sailor Saturn, the senshi with incredible destructive power - two opposites, practically.
Back to the manga - though they would never normally meet, fate drove them together and they found the reborn Saturn in Hotaru. There was always someone telling them to gather the talismans, but based on their past experience with the Silver Millennium, they felt that this would be a bad idea.
However, their efforts are in vain - Hotaru's body struggles with the strain of Mistress 9, her cyborg implants, and the impending awakening of Sailor Saturn. Mistress 9 eventually awakens from inside Hotaru and is able to call out to Pharaoh 90, who makes his way to Earth. Hotaru sacrifices her own life to save the Hostes of the Sailor Guardians and Chibiusa. All the Senshi reach their limits, and it seems that all hope is lost - Sailor Moon decides that the only way to destroy Pharaoh 90 is to gather all her power and release it inside of him.
As Sailor Moon is doing so, the three Talismans of the Outer Senshi begin to resonate and glow - and there is no doubt, this time, that it is Sailor Saturn's awakening and not the summoning of the Holy Grail. The Senshi watch as Sailor Saturn descends. The Goddess of Destruction, indeed - they are awed by her power as much as they fear it.
I, and the journeyer from the valley of the dead ... With the blessings of Saturn, the planet of destruction, I am the Scout of Silence ... Sailor Saturn. ~ Sailor Saturn (DUH), Act 33
With the ghost of the Silver Millennium still following her, it hasn't been very long since her last awakening, and with a wistful smile or perhaps even a smirk, she accepts that she is "always an uninvited guest." She sets off to work, beating Pharaoh 90 back, all the while perfectly calm, explaining the circumstances behind her awakening and the fate of the world.
She explains that there were several events that threw off the original path of history - Hotaru, who should have died in the accident at her father's lab, lived, straining her body and Saturn's sleeping soul. She was made into a Vessel, and her father called upon the aliens, drawn to Earth because it is a chosen land. Pharaoh 90 had come to the planet and everything was spiraling down into destruction, bought about by destiny, by Sailor Saturn. Thus, it was necessary for her to drop her Glaive.
Distraught, the Sailor Senshi (still sans Sailor Moon) watch as Saturn battles Pharaoh 90, powerless to stop what they think is the coming end of the world. Sailor Saturn drops her glaive, and the world begins to fall apart, but suddenly Sailor Moon reappears. Pharaoh 90 tries to escape to his home Tau system, and to the Senshi's shock, it is revealed that Saturn plans to follow him as the "guide to death." They are horrified, but Saturn reassures them.
Do not be distraught. Always with the end comes hope and rebirth. You are the one who brings that, Super Sailor Moon. ... Because you released the power of the legendary Moon Chalice and the Silver Imperium Crystal, this planet can be saved. I am the Scout who brings the death and destruction necessary for rebirth. ~ Sailor Saturn, Act 33
As Pharaoh 90 makes his getaway, Sailor Saturn follows him, telling Sailor Pluto to forever close the road to Pharaoh 90's dimension. Pluto is reluctant, having realized the true role of Saturn, but does as instructed, and Saturn is seen beyond the door, fading away with a smile on her face as she goes to sacrifice herself.
In the anime, the theme of "two Messiahs" is much more pronounced and discussed, with Sailor Moon and Sailor Saturn portrayed on opposite ends of the spectrum of good and evil. However, as the Senshi come to realize, both are necessary and neither is necessarily evil. Sailor Moon is the true Messiah, but without Saturn, the world could not have been saved.
Sailor Moon remembers Saturn's words, and as she restores Earth to what it once was, Hotaru is reborn onto the planet as a baby, with Saturn's symbol on her forehead. Uranus, Neptune, and Pluto find her, and promise to raise her as their own child.